whenever you delete a user all their posts turn into guest posts. AFAIK there is no way to get them back, as you don't know which guest posts were his.
the only way is if you had disabled guest posting, and every post was by a registered user. then you could update the database and make guest posts this user.
otherwise, they are just guest posts.
"A nerd is someone whose life is focused on computers and technology, but a geek is someone whose life is focused on computers and technology and LIKES it that way."